Dry-laid web with hollow synthetic fibers

ABSTRACT

The present invention relates to a dry-laid composition comprising: hollow synthetic fiber, absorbent and a binder system. The hollow fiber comprises from about 10 to about 50 percent by weight of said composition. The hollow synthetic fiber has a denier of between about 2 to about 18. The hollow synthetic fiber is selected from the class of polyolefins, polyesters, polyamides, acrylics, as well as mixtures and copolymers thereof. The absorbent comprises from about 40% to about 80% of the weight of said composition. The absorbent is a natural absorbent, or a synthetic absorbent, or a mixture of these. The natural absorbent is selected from the class of wood pulp fluff, cotton, cotton linters, and regenerated cellulose fibers, or a mixture of these. The synthetic absorbent is selected from the class of agar, pectin, guar gum, and synthetic hydrogel polymers. The binder fiber comprises from about 3 to about 15 percent by weight of said composition.

BACKGROUND OF THE INVENTION

[0001] 1) Field of the Invention

[0002] The present invention relates to a dry-laid composition useful in diapers, incontinent pads, sanitary napkins and other absorbent pads needed for body fluids. In particular, the present invention comprises a composition of absorbent, a binder system, and up to fifty percent hollow synthetic fibers based on the total composition weight. The composition of the present invention has an improved loft, compression resistance and fluid intake rate superior to existing composites based on natural absorbents and non-hollow synthetic fibers.

[0003] 2) Prior Art

[0004] Disposable absorbent articles such as disposable diapers, have found much success in the marketplace, however, there is always a need to improve these products in terms of their low density, high loft, compression resistance, and fluid intake rate. Prior to the present invention, it was known to form existing dry-laid composites from wood pulp (and optionally up to about 25% super absorbent polymer, SAP), bicomponent fibers as a binder, and synthetic fibers for loft and compression resistance. This existing composition contained approximately 10% bicomponent fibers, about 10% polyester fibers, and approximately 80% wood pulp. This product had adequate loft, fluid intake rate and good wet strength. Generally this product was created by mixing the wood pulp (and optionally the SAP), with the bicomponent fibers and the synthetic fibers. The dry-laid composite was then introduced into a heating zone, such that the lower melting material of the bicomponent fiber would melt and the molten lower melting material would run to the intersection where the fibers cross one another. Next, the composite was introduced into a cooling zone where the web was cooled, thus solidifying the molten lower melting material, thereby binding the mixture into a unitary web structure.

[0005] In this existing composition, the purpose of the wood pulp (and SAP, if present) is to absorb the body fluids, the purpose of the bicomponent fiber is to bind the entire web together and the purpose of the synthetic fibers, including the higher melting synthetic fiber component of the bicomponent fibers, is to provide loft and compression resistance so that the maximum surface of each individual wood pulp fiber may be exposed to the bodily fluids.

[0006] It is an object of the present invention to improve upon the loft, compression resistance, and the fluid intake rate. These features should be achieved without any loss of wet strength.

SUMMARY OF THE INVENTION

[0007] The present invention relates to a dry-laid composition comprising absorbent, binder, and hollow synthetic fibers. If the binder system is based on binder fibers, it is desired to maintain the percentage of binder at about 10% by weight as it has been determined that this amount of binder is sufficient to adequately bind the web into a unitary structure. On the other hand, the hollow fiber can comprise from about 10% up to about 50% by weight of the composition. This means that the absorbent comprise from 40% to 80% by weight of the composition. Replacing the solid synthetic fibers with synthetic hollow fibers surprisingly increases the loft and the fluid intake rate as well as the compression resistance.

[0008] In the broadest sense, the present invention relates to a dry-laid composite of absorbent, a binder system, and hollow synthetic fibers.

[0009] Dry-laid composites of the present invention provide good compression resistance, loft, and fluid intake increase as compared with a composite based on bicomponent fibers, wood pulp fibers, and solid synthetic fibers. To increase the absorbency of the web of the present invention, some of the wood pulp may be replaced with SAP.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Dry-laid webs of the present invention are webs made not using water or other liquids. Such webs are made by the air-lay, carding, garneting, or random carding processes. Air-laid webs are created by introducing the fibers into an air current, which uniformly mixes the fibers and then deposits them on a screen surface. The carding process separates tufts into individual fibers by combing or raking the fibers into a parallel alignment. Garnetting is similar to carding in that the fibers are combed. Thereafter the combed fibers are interlocked to form a web. Multiple webs can be overlapped to build up a desired weight. Random carding uses centrifugal force to throw fibers into a web with random orientation of the fibers. Again multilayers can be created to obtain the desired web weight.

[0011] The web of fibers can be bonded by mechanical, chemical or thermal means. Mechanical bonding uses entanglements introduced by needle punching or hydroentangling. If no other binder system is employed, the web comprises absorbent and hollow fibers. Generally, mechanical bonding is inadequate for most uses of the web of the present invention. Chemical bonding uses adhesives such as latex resins, or hot melt adhesives. Thermal bonding utilizes binder fibers in an oven (hot air, radiant or microwave), or on heated calender roll(s), or by ultrasonic energy.

[0012] If the binder system and absorbent are in the form of fibers, these fibers can merely be added to the synthetic fibers when the dry laid webs are created according to any of the processes described above. This has the added benefit that a screen conveyor belt can transport the mixed components to a heating zone, which causes the binder fiber to become molten, and then to a cooling zone where the molten binder resolidifies. The web now has sufficient rigid structure to be useful as a component of an absorbent pad.

[0013] Suitable absorbents are natural or synthetic absorbents. Synthetic absorbents are primarily known as super absorbent polymers (SAP). The absorbents comprise 40-80% by weight of the web. Natural absorbents are hydrophilic materials such as cellulosic fibers, wood pulp fluff, cotton, cotton linters, and regenerated cellulose fibers such as rayon, or a mixture of these. Preferred is wood pulp fluff, which is both inexpensive and readily available.

[0014] Conventional natural absorbents do not absorb as much bodily fluid as when a portion of them has been replaced with synthetic fibers, and preferably polyester fibers, which provide loft to the composite. Providing loft to the composite exposes more surface area of the natural absorbents to the bodily fluids and thus they are much more efficient in absorbing the bodily fluid.

[0015] Absorbent pads employing natural absorbents may not provide adequate fluid intake for all circumstances. Also natural absorbents are very bulky. Accordingly, many absorbent pads employ SAP in relatively low quantities. This is because the cost of SAP is much higher than the cost of natural absorbents. Replacing some of the natural absorbents with SAP can reduce the overall bulk of the pad and/or provide superior fluid intake.

[0016] As used herein, the term “super absorbent polymer” or “SAP” refers to a water-swellable, generally water-insoluble material capable of absorbing at least about 10, desirably about 20, and preferably about 50 times or more its weight in water. The super absorbent polymer may be formed from organic material, which may include natural materials such as agar, pectin, and guar gum, as well as synthetic materials such as synthetic hydrogel polymers. Synthetic hydrogel polymers include, for example, carboxymethyl cellulose, alkali metal salts of polyacrylic acid, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, polymers and copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinyl pyridine, and the like. Other suitable polymers include hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, and isobutylene maleic anhydride copolymers and mixtures thereof. The hydrogel polymers are preferably lightly crosslinked to render the materials substantially water insoluble. Crosslinking may, for example, be by irradiation or covalent, ionic, van der Waals, or hydrogen bonding. Suitable materials are available from various commercial vendors such as the Dow Chemical Company, Allied Colloid, Inc., and Stockhausen, Inc. The super absorbent polymer may be in the form of particles, flakes, fibers, rods, films or any of a number of geometric forms.

[0017] The synthetic hollow fibers not only improve the loft of the composite compared to a composite having solid synthetic fibers, but the fluid intake rate is increased perhaps due to the capillary effect of the fluid being drawn into the hollow fiber. In other words, capillary forces draw the bodily fluid into the hollow synthetic fiber. Generally, the fibers are between 3 and 65 mm in length and preferably from about 4 to 40 mm in length. Preferably all the fibers are hollow. However a portion of them can be replaced with any other solid polymeric fibers. The synthetic hollow fibers comprise 10-50% by weight of the web. The web composition of the present invention has weights in the range of about 50 to about 500 grams per square meter (gsm).

[0018] The synthetic hollow fibers may be formed from any polymeric material capable of forming fibers which fibers can be formed into a fibrous web. Suitable polymeric material, from which the synthetic hollow polymeric fibers may be formed, include polyolefins, such as polyethylene, polypropylene, and the like; polyesters such as polyethylene terephthalate or polybutylene terephthalate, or copolyesters such as polyethylene terephthalate-isophthalate or polyethylene terephthalate-adipate and the like; polyamides such as nylon 6, nylon 6,6, poly (iminocarboxylpentamethylene) and the like; acrylics, as well as mixtures and copolymers thereof. Preferred is polyester fiber such as polyethylene terephthalate.

[0019] The synthetic hollow fibers may be formed by a conventional melt spinning, drawing, crimping and cutting process into staple fibers having a length, for example, of from about 25 millimeters to about 75 millimeters or short cut into lengths of from about 1 millimeter to about 25 millimeters. The synthetic polymeric fibers may suitably have a maximum cross-sectional dimension of from about 10 micrometers to about 50 micrometers as determined by microscopic measurement using an optical microscope and a calibrated stage micrometer or by measurement from Scanning Electron photomicrographs.

[0020] The webs can be bonded by the normal methods, but mechanical bonding is less preferred. The preferred binder systems of the present invention are conventional latex systems, hot melt adhesives, or binder fibers, or a mixture of these. Conventional latex systems such as styrene-butadiene copolymer, acrylate, and polyvinyl acetate systems, as well as mixtures of these are well known. When a conventional latex system is employed with the present invention, the amount of binder may range from 5-60% by weight of the web. Hot melt adhesives are generally solid powder materials or non-latex paste and liquid compositions well known to those in the art. Binder fibers can be conventional low melt fibers or bicomponent fibers. Conventional low melt fibers can be polyolefins, for example, and in particular can be linear low-density polyethylene. Bicomponent fibers having a denier of between 2 and 6 are the preferred binder fiber. Bicomponent fibers can be of the type in which the low melting point portion is adjacent to the high melting point portion such as a side-by-side configuration, or in a sheath-core configuration wherein the sheath is the lower melting component and the core is the higher melting component. The binders are thermally bonded by conventional means such as by using an oven (hot air, radiant or microwave), or calender roll(s), or by ultrasonic energy. It is contemplated that the web of the present invention will comprise between 3 and 15% by weight binder fiber, such as bicomponent fiber. This amount of binder fiber is deemed to be adequate to bind the web into a unitary structure. Preferably, about 10% by weight binder fiber (based on the weight of the web) gives most satisfactory results.

[0021] Suitable bicomponent fibers have a denier of between about 2-18 and can comprise polyethylene/polypropylene; polyethylene/polyester (especially polyethylene terephthalate); polypropylene/polyester; copolyester/polyethylene terephthalate, such as polyethylene terephthalate-isophthalate/polyethylene terephthalate; nylon 6/nylon 6,6; and nylon 6/polyethylene terephthalate. Preferably polyethylene/polyester are used, especially grafted polyethylene/polyethylene terephthalate, such as linear low-density polyethylene/polyethylene terephthalate.

[0022] The web may be formed by any of the dry-lay processes previously mentioned. If binder fibers are employed, a screen conveyor belt conveys the web to a heated zone of sufficient temperature and having a sufficient residence time such that the low melting material of the binder fiber melts, flows to the intersection of a group of overlaid, contacting and intersecting fibers. Next, the web is transported on the conveyor belt to a cooling zone where all the molten material solidifies thus making the web structurally rigid. If the binder is a latex system, the web is coated with the latex (by spraying, dipping, etc) and the latex is allowed to dry and cure, thereby solidifying. Thereafter, the web may be cut into various lengths and widths for the end use applications, namely, fenestration drapes, dental bibs, eye pads, diapers, incontinent pads, sanitary napkins, wet wipes, and wound dressing pads.

[0023] The web can also be used in conjunction with other components, such as part of a laminate with a woven or nonwoven material or fabric. For example, spun-bond/melt-blown/spun-bond fabric (“SMS”) are known and have many uses. Stitch-bonded fabric is another known example of a multicomponent structure comprising fabric and a fiber web sewn or stitched together.

Test Procedures

[0024] The properties of the polyester fibers and webs were measured according to the following procedures.

[0025] Loft

[0026] The loft under various loads was measured with a tensile tester having a pressure foot with an area of 50 sq. in. Crosshead speed was set at 24 in./minute.

[0027] Four non-woven sheets were stacked on each other and placed on the crosshead table. The crosshead was raised until the pressure foot comes in contact with the stack of non-woven sheets, the thickness was measured and reported as the initial loft (L_(i) inch). The crosshead was raised, and stopped for 30 seconds, at each of the following loads, 0.5, 2, 4, 10, 15, 25 and 50 lb. and the thickness measured at each load. After 5 minutes under a load of 50 lb., the crosshead was then lowered until the pressure foot is completely clear from the non-woven stack. After allowing the sample to relax for 5 minutes, the crosshead was raised until a load of 0.5 lb. was indicated, and the thickness measured (L₁ in.). The thickness under the initial load of 0.5 lb. (L₀ in.) is used in the calculation of % recovery, rather than the initial loft (L_(i)), to eliminate any variations in thickness due to the stacking of the sheets.

[0028] The percent recovery is

(L₁/L₀)×100

[0029] Web Strength

[0030] The wet and dry strength of the web was measured according to TAPPI test methods T 456 om-87 and T 494 om-88 respectively. The wet strength being measured after an immersion time of 15 sec.

[0031] Liquid Acquisition Test

[0032] An 8-centimeter (cm) diameter sample is supported at the bottom of a 8 cm. diameter cylinder, such that the liquid that flows through the sample is collected on one balance, and the liquid that overflows from the top of the sample is collected on another balance. 100 ml of a 0.9% saline solution is poured onto the surface of the sample at a nominal flow rate of 7 ml/sec. The weight of the liquid that leaks and overflows is recorded at 2 second intervals.

[0033] Acquisition Rate

[0034] A 6 in. (15.2 cm) square of the web is placed horizontally between 2 clamps. 100 ml of water is poured onto the center of the sample at a rate of 20 ml/sec. The time (t in sec.), from the completion of the pouring, for the water to disappear from the surface of the web is recorded. The acquisition rate (AR, ml/sec.) is:

AR=100/t

EXAMPLES

[0035] All webs for these examples were prepared with the same basis weight of 263 gsm, and all the polyester fibers were of 6 mm length. The bicomponent fiber (10% of the web composition) was KoSa Type 255, 3 den (denier), 6 mm length.

Example 1

[0036] In Example 1, four different web compositions were prepared by means of the air-laid procedure described previously, in which the amount of polyester fiber varied from 0 to 40% by weight of the total web and varying the denier of polyester fiber and its cross-section (solid or hollow).

[0037] Where the data indicates that there is no polyester means that the composition of the web comprised 10% bicomponent fibers and 90% wood pulp fibers. The results are set forth in Table 1, which shows the improved initial loft using a hollow fiber at a 40% level. TABLE 1 Initial loft (no loading), in. % PET 0 10 20   40   Solid 3 den 1.4 1.6 — — Solid 6 den 1.4 1.6 1.6 1.7 Hollow 6 den 1.4 1.6 1.8 2.3 Solid 15 den 1.4 1.6 — —

[0038] Table 2 shows the improvement in loft under compression with all webs containing additional polyester fibers at the 10% loading. TABLE 2 Loft under compression, in. (10% PET) Load, lb 0 0.5 2 4 10 15 25 No PET 1.4 1.3 1.2 1.1 0.9 0.7 0.5 Solid 3 den 1.6 1.5 1.3 1.2 1 0.9 0.8 Solid 6 den 1.6 1.5 1.3 1.2 1 0.95 0.85 Solid 15 den 1.6 1.5 1.4 1.3 1.1 1 0.9 Hollow 6 den 1.6 1.5 1.3 1.2 1 0.9 0.8

[0039] Tables 3 and 4 show the improvement in loft under compression at a 20% and 40% loading of a hollow 6 denier fiber compared to a 6 denier solid fiber. TABLE 3 Loft under compression, in. (20% PET-hollow effect) Load, lb 0 0.5 2 4 10 15 25 No PET 1.4 1.3 1.2 1.1 0.9 0.7 0.5 Solid 6 den 1.6 1.4 1.2 1.1 0.9 0.8 0.7 Hollow 6 den 1.8 1.7 1.5 1.4 1.1 1 0.8

[0040] TABLE 4 Loft under compression, in. (40% PET-hollow effect) Load, lb 0 0.5 2 4 10 15 25 No PET 1.4 1.3 1.2 1.1 0.9 0.7 0.5 Solid 6 den 1.7 1.6 1.4 1.3 1.1 1 0.85 Hollow 6 den 2.3 2 1.8 1.6 1.4 1.2 1.1

[0041] Tables 5 and 6 illustrate the better recovery from compression with higher denier fibers, and with hollow fibers at the same denier. TABLE 5 % Recovery No PET 81 10% solid 3 den 83 10% solid 15 den 88

[0042] TABLE 6 % Recovery No PET 81 40% solid 6 den 87 40% hollow 6 den 89

Example 2

[0043] The web wet/dry strength ratio in the machine direction (MD) and cross direction (CD) of various webs was compared. The first web has no synthetic fiber (no PET), the second and third webs have solid fiber and the last web has hollow fiber. The solid and hollow fiber webs contained 10% bicomponent, 80% wood pulp and 10% polyester (of either 3 denier solid, 15 denier solid, or 6 denier hollow). The results are set forth in Table 7. TABLE 7 Wet/Dry Strength (%) MD wet/MD dry MD wet/CD dry No PET 0.40 0.42 10% solid 3 den 0.60 0.62 10% solid 15 den 0.59 0.61 10% hollow 6 den 0.60 0.63

[0044] The results show that webs having synthetic fibers are an improvement over the web with no PET, while the web with hollow fiber had results comparable to solid fiber webs.

Example 3

[0045] The liquid acquisition test compared 6 den solid PET with 6 den hollow PET. The results are set forth below in Tables 9 and 10, and illustrate that the hollow fiber prevented any overflow. The most desirable behavior is to have instant penetration—no overflow and little or no leakage. Failing that, one wants as little overflow as possible, and to have the onset of overflow delayed as long as possible. TABLE 9 Overflow, ml Time, second 0 2 4 6 8 10 12 14 16 18 Solid PET 0 0 0 0 6 13 21 29 34 34 Hollow PET 0 0 0 0 0 0 0 0 0 0

[0046] As the results show, the hollow fiber web had no overflow even after 18 seconds, indicating instant liquid penetration. This is a very desirable result for disposable diapers, for example. TABLE 10 Leakage, ml Time, second 0 2 4 6 8 10 12 14 16 18 Solid PET 0 0 0 0 3 10 17 24 30 34 Hollow PET 0 0 0 0 2 13 26 40 52 53

[0047] Comparing the total of Overflow and Leakage, the hollow fiber webs are superior to solid fiber webs.

[0048] Table 11 shows the improved acquisition rate of these webs. The higher the acquisition rate the faster the web permits liquid to flow through it. This means that bodily fluids do not remain in contact with the skin for a long period of time, another desirable feature for disposable diapers and incontinent pads. Hollow fiber webs have a rate double that of solid fiber webs. TABLE 11 Acquisition Rate (ml/sec) No PET 2 Solid PET 3 Hollow PET 7

[0049] Thus it is apparent that there has been provided, in accordance with the invention, a dry-laid composition that fully satisfies the objects, aims, and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations would be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the invention. 

What is claimed is:
 1. A dry-laid composition comprising: hollow synthetic fiber, absorbent and a binder system.
 2. The composition of claim 1, wherein said hollow fiber comprises from about 10 to about 50 percent by weight of said composition.
 3. The composition of claim 1, wherein said hollow synthetic fiber has a denier of between about 2 to about
 18. 4. The composition of claim 1, wherein said hollow synthetic fiber is selected from the class of polyolefins, polyesters, polyamides, acrylics, as well as mixtures and copolymers thereof.
 5. The composition of claim 4, wherein said polyester is polyethylene terephthalate.
 6. The composition of claim 1, wherein said binder system is selected from the class of mechanical, chemical, or binder fiber, or a mixture of these.
 7. The composition of claim 6, wherein said mechanical bonding is by needle punching or hydroentangling.
 8. The composition of claim 6, wherein said chemical bonding is by latex resins or hot melt adhesives.
 9. The composition of claim 6, wherein said binder fiber employs low melt polymer fibers or bicomponent fibers
 10. The composition of claim 9, wherein said binder fiber comprises from about 3 to about 15 percent by weight of said composition.
 11. The composition of claim 9, wherein suitable bicomponent fibers are selected from the class of polyethylene/polypropylene; polyethylene/polyester; polypropylene/polyester; copolyester/polyethylene terephthalate; nylon 6/nylon 6,6; and nylon 6/polyethylene terephthalate.
 12. The composition of claim 11, wherein said polyethylene/polyester is a grafted polyethylene/polyethylene terephthalate.
 13. The composition of claim 1, wherein said absorbent comprises from about 40% to about 80% of the weight of said composition.
 14. The composition of claim 1, wherein said absorbent is a natural absorbent, or a synthetic absorbent, or a mixture of these.
 15. The composition of claim 14, wherein said natural absorbent is selected from the class of wood pulp fluff, cotton, cotton linters, and regenerated cellulose fibers, or a mixture of these.
 16. The composition of claim 14, wherein said synthetic absorbent is selected from the class of agar, pectin, guar gum, and synthetic hydrogel polymers.
 17. The composition of claim 16, wherein said synthetic hydrogel polymers are selected from the class of carboxymethyl cellulose, alkali metal salts of polyacrylic acid, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, polymers and copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinyl pyridine, and mixtures of these.
 18. A process of making a dry-laid web comprising: mixing hollow fibers, absorbent, and binder fibers; depositing said mixture onto a moving conveyor belt; heating said deposited mixture to a temperature sufficient to melt said binder fibers; and cooling said web thereby forming a structurally rigid web.
 19. The process of claim 18, wherein said hollow fiber comprises from about 10 to about 50 percent by weight of said web.
 20. The process of claim 18, wherein said binder fibers are bicomponent fibers.
 21. The process of claim 18, wherein said binder fibers comprise from about 3 to about 15 percent by weight of said web.
 22. The process of claim 18, wherein said absorbent comprises from about 40 percent to about 80 percent of the weight of said web.
 23. A process of making a rigid dry-laid web comprising: mixing hollow fibers and absorbent fibers; depositing said mixture onto a surface thereby creating a loose web; bonding said loose web thereby creating a rigid unitary web.
 24. The process of claim 23, wherein said hollow fibers comprise from about 10 to about 50 percent by weight of said web. 